If you like to tinker with your own system, that’s fine with us. However, if you change things like stylesheets and icons, you should be aware that you’re in unsupported territory. Any issues you encounter should be reported to the theme developer, not the app developer.
I don’t know whether you’re shitting on theme developers or GTK app developers with your comment, but they explicitly state that they think theming is fine, they’re just tired of people reporting theme problems as app problems. It’s a completely reasonable take.
If I were an app developer I wouldn’t want to open a bug report, then spend hours and hours investigating a reported issue, only to find out that my app was never the problem in the first place.